large crabgrass vs crab grass

large crabgrass and crab grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
large crabgrass	Yes	No	No	No
crab grass	Yes	No	No	No

As nouns, crab grass is a hypernym of large crabgrass; that is, crab grass is a word with a broader meaning than large crabgrass:

large crabgrass: a European forage grass grown for hay; a naturalized weed in United States
crab grass: grasses with creeping stems that root freely; a pest in lawns

Other hypernyms of large crabgrass include crabgrass, finger grass.

large crabgrass (noun)	crab grass (noun)
a European forage grass grown for hay; a naturalized weed in United States	grasses with creeping stems that root freely; a pest in lawns